Showing papers on "Aircraft noise published in 1983"

Conventional profile coaxial jet noise prediction

Abstract: A semiempirical model for predicting the noise generated by conventional velocity-profile jets exhausting from coaxial nozzles is presented and compared with small-scale static data. The present method is an updated version of that part of the original NASA Aircraft Noise Prediction Program (1974) relating to coaxial jet noise. The new procedure is more theoretically based and has also been improved by some empirical adjustments.

Aircraft Turbofan Noise

Abstract: Turbofan noise generation and suppression in aircraft engines are reviewed. The chain of physical processes which connect unsteady flow interactions with fan blades to far field noise is addressed. Mechanism identification and description, duct propagation, radiation and acoustic suppression are discussed. The experimental technique of fan inflow static tests are discussed. Rotor blade surface pressure and wake velocity measurements aid in the determination of the types and strengths of the generation mechanisms. Approaches to predicting or measuring acoustic mode content, optimizing treatment impedance to maximize attenuation, translating impedance into porous wall structure and interpreting far field directivity patterns are illustrated by comparisons of analytical and experimental results. The interdependence of source and acoustic treatment design to minimize far field noise is emphasized. Area requiring further research are discussed and the relevance of aircraft turbofan results to quieting other turbomachinery installations is addressed.

A preliminary comparison between the SR-6 propeller noise in flight and in a wind tunnel

Abstract: High speed turboprops offer an attractive candidate for aircraft because of their high propulsive efficiency. However, one of the possible problems associated with these propellers is their high noise level at cruise condition that may create a cabin environment problem. Models of these propellers were tested for acoustics in the 8 by 6-foot wind tunnel and on the Jet Star airplane. Comparisons between the airplane and wind tunnel data for the SR-6 propeller are shown. The comparison of maximum blade passing tone variation with helical tip Mach number between the tunnel and flight data was good when corrected to the same test conditions. Directivity comparisons also showed fairly good agreement. These good comparisons indicate that the wind tunnel is a viable location for measuring the blade passage tone noise of these propellers.

Analytical model of jet shielding

Abstract: An analytical model of the shielding of a stationary point noise source by a cylindrical jet is developed. The directivity function is derived which estimates the normalized sound pressure level at a far field receiver. The shielding model is compared to experimental data for a point noise source impinging on an unheated air jet and on a simulated hot air jet. The model compares favorably to measured shielding at receiver locations away from the jet axis. The trend of the estimated shielding diverges from the measured data as the jet axis is approached. Refinement of the model is discussed. STIMATION of aircraft generated noise includes identification not only of the sources of noise on the aircraft, but also of the propagation path between the source and receiver. One of the numerous factors affecting the noise transmission path is shielding of one jet by another. The shielding jet, because of the high temperature and flow speed with respect to the immediate surroundings, acts as a partial barrier between the source and the receiver. The resultant noise reduction not only affects the overall aircraft noise level, but also indicates the possibility of jet engine in- stallation as a means of aircraft noise control. The problem of reflection and transmission of sound by a moving medium has been addressed assuming a plane wave incident on a plane surface.1"4 Ray tracing techniques have been applied to two-dimensional jets5 and cold jets.6 The two- dimensional formulation, for a cylindrical noise source impinging on the shielding jet has been developed.7 The present study is an extension to three dimensions of the previous analysis.7 The model developed consists of the sound field emitted from a stationary, discrete frequency point source, which impinges on a cylinder of locally parallel flow. The temperature and velocity profiles are uniform in the jet at any location downstream of the nozzle. While this model is an idealization of the twin jet, it is felt to incorporate the basic elements essential for a realistic representation; not only of the source, but also of the shielding jet. Model Development Formulation of the Model The mechanisms by which shielding occurs are reflection of sound at the boundary between the jet and the surrounding air and by diffraction around the jet. The noise source is modeled by a stationary discrete frequency point source located at (r0, 0,0). The shielding jet is a cylinder of radius a, and is infinite in extent along the z axis. The temperature and flow velocity are uniform across the cylinder cross section. The model is illustrated in Fig. 1. The expression for acoustic velocity potential is written for two regions; region I is outside the jet, region II is within the jet. In region I (outside the flow) In region II (inside the flow)

Application of 3-signal coherence to core noise transmission

Abstract: A method for determining transfer functions across turbofan engine components and from the engine to the far-field is developed. The method is based on the three-signal coherence technique used previously to obtain far-field core noise levels. This method eliminates the bias error in transfer function measurements due to contamination of measured pressures by nonpropagating pressure fluctuations. Measured transfer functions from the engine to the far-field, across the tailpipe, and across the turbine are presented for three turbofan engines.

Interior noise considerations for advanced high-speed turboprop aircraft

Abstract: This paper describes recent research on noise generated by high-speed propellers, on noise transmission through acoustically treated aircraft sidewalls, and on subjective response to simulated turboprop noise. Propeller noise discussion focuses on theoretical prediction methods for complex blade shapes designed for low noise at Mach = 0.8 flight and on comparisons with experimental test results. Noise transmission experiments using a 168-cm-diam aircraft fuselage model and scaled heavy-double-wall treatments indicate that the treatments perform well and that the predictions are usually conservative. Studies of subjective comfort response in an anechoic environment are described for noise signatures having combinations of broadband and propellertype tone components.

Design of sidewall treatment of cabin noise control of a twin engine turboprop aircraft

Abstract: An analytical procedure was used to predict the noise transmission into the cabin of a twin engine general aviation aircraft. This model was then used to optimize the interior A weighted noise levels to an average value of about 85 dBA. The surface pressure noise spectral levels were selected utilizing experimental flight data and empirical predictions. The add on treatments considered in this optimization study include aluminum honeycomb panels, constrained layer damping tape, porous acoustic blankets, acoustic foams, septum barriers and limp trim panels which are isolated from the vibration of the main sidewall structure. To reduce the average noise level in the cabin from about 102 kBA (baseline) to 85 dBA (optimized), the added weight of the noise control treatment is about 2% of the total gross takeoff weight of the aircraft.

A study of interior noise levels, noise sources and transmission paths in light aircraft

Abstract: The interior noise levels and spectral characteristics of 18 single-and twin-engine propeller-driven light aircraft, and source-path diagnosis of a single-engine aircraft which was considered representative of a large part of the fleet were studied The purpose of the flight surveys was to measure internal noise levels and identify principal noise sources and paths under a carefully controlled and standardized set of flight procedures The diagnostic tests consisted of flights and ground tests in which various parts of the aircraft, such as engine mounts, the engine compartment, exhaust pipe, individual panels, and the wing strut were instrumented to determine source levels and transmission path strengths using the transfer function technique Predominant source and path combinations are identified Experimental techniques are described Data, transfer function calculations to derive source-path contributions to the cabin acoustic environment, and implications of the findings for noise control design are analyzed

Noise monitoring in the vicinity of general aviation airports

Abstract: In order to examine issues related to noise monitoring in the vicinity of moderate‐sized airports, monitoring was performed for 5 months at three sites in the vicinity of the Decatur, Illinois Municipal Airport. The study issues included (1) comparison of predicted and measured day/night average sound levels (DNL), (2) comparison of individual aircraft levels with prediction, (3) temporal sampling requirements for monitoring, and (4) validation that the measured noise is aircraft noise and not other community noise. The measured DNL and the computer predictions compare quite favorably, and most individual aircraft levels compare favorably with data in the FAA computer models. In terms of temporal sampling requirements, it appears that one can use a strategy of choosing four random weeks throughout the year (one from each quarter) to achieve a +2 to −3‐dB tolerance. It was not possible to differentiate ‘‘airport’’ noise from other ‘‘community’’ noise. The results show that it may be better (and far less co...

Development of rotorcraft interior. Noise control concepts. Phase 1: Definition study

Abstract: A description of helicopter noise, diagnostic techniques for source and path identification, an interior noise prediction model, and a measurement program for model validation are provided

In-flight acoustic testing techniques using the YO-3A Acoustic Research Aircraft

Abstract: This report discusses the flight testing techniques and equipment employed during air-to-air acoustic testing of helicopters at Ames Research Center. The in flight measurement technique used enables acoustic data to be obtained without the limitations of anechoic chambers or the multitude of variables encountered in ground based flyover testing. The air-to-air testing is made possible by the NASA YO-3A Acoustic Research Aircraft. This "Quiet Aircraft' is an acoustically instrumented version of a quiet observation aircraft manufactured for the military. To date, tests with the following aircraft have been conducted: YO-3A background noise; Hughes 500D; Hughes AH-64; Bell AH-1S; Bell AH-1G. Several system upgrades are being designed and implemented to improve the quality of data. This report will discuss not only the equipment involved and aircraft tested, but also the techniques used in these tests. In particular, formation flying position locations, and the test matrices will be discussed. Examples of data taken will also be presented.

Propeller noise prediction

Abstract: Analytic propeller noise prediction involves a sequence of computations culminating in the application of acoustic equations. This paper describes the prediction sequence currently used by NASA in its ANOPP (Aircraft Noise Prediction) program. No attempt is made here to review the state of the art of noise prediction. Some elements of this sequence represent classic results while other represent the most recent publications. The elements of the sequence are called program modules. The first group of modules analyze the propeller geometry, the aerodynamics, including both potential and boundary layer flow, the propeller performance, and the surface loading distribution. This group of modules is based entirely on aerodynamic strip theory. The next group of modules deals with the actual noise prediction, based on data from the first group. Deterministic predictions of periodic thickness and loading noise are made using Farassat's time‐domain methods. Broadband noise is predicted by the semi‐emipirical Schlinker‐Amiet method. Nearfield predictions of fuselage surface pressures include the effects of boundary layer refraction and (for a cylinder) scattering. Farfield predictions include atmospheric and ground effects. Comparisons are made to experimental data from subsonic and transonic propellers and NASA's future directions in propeller noise technology development are indicated.

Limits on the prediction of helicopter rotor noise using thickness and loading sources: Validation of helicopter noise prediction techniques

Abstract: The techniques of helicopter rotor noise prediction attempt to describe precisely the details of the noise field and remove the empiricisms and restrictions inherent in previous methods These techniques require detailed inputs of the rotor geometry, operating conditions, and blade surface pressure distribution The Farassat noise prediction techniques was studied, and high speed helicopter noise prediction using more detailed representations of the thickness and loading noise sources was investigated These predictions were based on the measured blade surface pressures on an AH-1G rotor and compared to the measured sound field Although refinements in the representation of the thickness and loading noise sources improve the calculation, there are still discrepancies between the measured and predicted sound field Analysis of the blade surface pressure data indicates shocks on the blades, which are probably responsible for these discrepancies

A laboratory study of the perceived benefit of additional noise attenuation by houses

Abstract: Two Experiments were conducted to investigate the perceived benefit of additional house attenuation against aircraft flyover noise. First, subjects made annoyance judgments in a simulated living room while an operative window with real and dummy storm windows was manipulated in full view of those subjects. Second, subjects made annoyance judgments in an anechoic audiometric test chamber of frequency shaped noise signals having spectra closely matched to those of the aircraft flyover noises reproduced in the first experiment. These stimuli represented the aircraft flyover noises in levels and spectra but without the situational and visual cues present in the simulated living room. Perceptual constancy theory implies that annoyance tends to remain constant despite reductions in noise level caused by additional attenuation of which the subjects are fully aware. This theory was supported when account was taken for a reported annoyance overestimation for certain spectra and for a simulated condition cue overreaction.

Aircraft trajectories for reduced noise impact

Abstract: Numerical optimization is used to compute the optimum flight paths, based upon a parametric form that implicitly includes some of the problem restrictions. The other constraints are formulated as penalties in the cost function. Various aircraft on multiple trajectories (landing and takeoff) can be considered. The modular design employed allows for the substitution of alternate models of the population distribution, aircraft noise, flight paths, and annoyance, or for the addition of other features (e.g., fuel consumption) in the cost function. A reduction in the required amount of searching over local minima was achieved through use of the presence of statistical lateral dispersion in the flight paths. Previously announced in STAR as N81-31159

Farfield inflight measurements of high-speed turboprop noise

Abstract: A flight program was carried out to determine the variation of noise level with distance from a model high speed propeller. Noise measurements were obtained at different distances from a SR-3 propeller mounted on a JetStar aircraft, with the test instrumentation mounted on a Lear jet flown in formation. The propeller was operated at 0.8 flight Mach number, 1.12 helical tip Mach number and at 0.7 flight Mach number, 1.0 helical tip Mach number. The instantaneous pressure from individual blades was observed to rise faster at the 0.8 M flight speed, than at the 0.7 M flight speed. The measured levels appeared to decrease in good agreement with a 6 dB/doubling of distance decay, over the measurement range of approximately 16 m to 100 m distance. Further extrapolation, to the distances represented by a community, would suggest that the propagated levels during cruise would not cause a serious community annoyance.

Operational military helicopter interior noise and vibration measurements with comparisons to ride quality criteria

Abstract: Balka (1981) has identified the attainment of a 'jet-smooth' ride as a primary goal of the helicopter industry for commercial and certain military helicopters. It was noted that criteria accounting for both multiple axis vibration and interior noise are needed. The present investigation has the objective to present a vibration and interior noise data base in a format suitable for direct evaluation of aircraft ride quality. The investigation is also concerned with an assessment of the measured environment against available criteria as an indication of the state-of-the-art for current machines. Interior noise and vibration measurements were obtained on eight military helicopters during routine operational flights. The data are presented in the form of a number of parameters.

Lateral attenuation of aircraft noise

Abstract: Lateral attenuation of aircraft noise comprises all of the losses in addition to spherical spreading and at- mospheric absorption. The phenomenon is primarily due to ground interference effects and is often regarded as a function of source-receiv er distance and elevation angle. In this paper, theoretical predictions are made in order to examine the consistency of existing empirical data on lateral noise attenuation. The results indicate that the effects of source spectrum shape and meteorological conditions must also be considered in any model for predicting lateral noise attenuation. F great importance to the control of community noise around airports is an accurate knowledge of the locations of equal noise load contours. In turn, the noise load at a given position is strongly dependent on the noise levels received from the individual aircraft at that place. To obtain the noise levels at arbitrary positions, ex- trapolations are usually made from noise data measured beneath the flight path. Although the noise level decay rate with increasing distance from the source is due primarily to spherical spreading and atmospheric absorption, other phenomena also produce attenuations. In particular, excess attenuations are caused by the effects of ground reflection. In addition, at low elevation .angles, shielding effects of the aircraft on emitted noise might alter the observed noise levels. The latter influences give rise to the so-called lateral noise attenuation. By definition, this quantity is the difference in time-integrat ed or maximum noise levels during flyover between that received on the flight track and that received at a sideline position, at a distance from the aircraft equal to the distance to the position on the flight track. The SAE-A21 Aircraft Noise Committee has developed an interim empirical prediction method for lateral attenuation of aircraft noise.1 The method uses the effective perceived noise level as a measure of the noise. The recommended curves for

Measurements and predictions of turboprop noise at high cruise speed

Abstract: Acoustic tests of advanced turboprop (Prop-Fan) models have taken place in or on three facilities: an acoustic wind tunnel, an aerodynamic wind tunnel, and a flight vehicle. Comparisons of data from the three facilities are made for two model designs, the unswept SR-2 and the swept SR-3. The importance of noise propagation through the boundary layer to microphones mounted on the surface of a flight vehicle is discussed and limited comparisons between predictions and measurements are shown. It is concluded that existing methodology is adequate for predictions at high cruise Mach number and that further development is required to improve the methodology for predicting fuselage and wind tunnel boundary layer propagation effects.

Comparison of broadband noise mechanisms, analyses, and experiments on helicopters, propellers, and wind turbines

Abstract: Experimental data on broadband noise from airfoils are compared, together with analytical methods, in order to identify the mechanisms of noise emission. Rotor noise is categorized into discrete frequency, impulsive, and broadband components, the last having a continuous spectrum originating from a random source. The results of computer simulations of different rotor blade types which produce broadband noise were compared with experimental data and among themselves in terms of predictions of the spectra obtained. Consideration was given to the overall sound pressure level, unsteady turbulence forces, rotational forces, inflow turbulence, self-generated turbulence, and turbulence in the flow. Data are presented for a helicopter rotor and light aircraft propeller. The most significant source was found to be inflow turbulence induced lift fluctuations in helicopter rotors and boundary layer trailing edge noise on large wind energy conversion systems

A pilot study of human response to general aviation aircraft noise

Abstract: A pilot study, conducted to evaluate procedures for measuring the noise impact and community response to general aviation aircraft around Torrance Municipal Airport, a typical large GA airport, employed Torrance Airport's computer-based aircraft noise monitoring system, which includes nine permanent monitor stations surrounding the airport. Some 18 residences near these monitor stations were equipped with digital noise level recorders to measure indoor noise levels. Residents were instructed to fill out annoyance diaries for periods of 5-6 days, logging the time of each annoying aircraft overflight noise event and judging its degree of annoyance on a seven-point scale. Among the noise metrics studied, the differential between outdoor maximum A-weighted noise level of the aircraft and the outdoor background level showed the best correlation with annoyance; this correlation was clearly seen at only high noise levels, And was only slightly better than that using outdoor aircraft noise level alone. The results indicate that, on a national basis, a telephone survey coupled with outdoor noise measurements would provide an efficient and practical means of assessing the noise impact of general aviation aircraft.

Computer Programs for Producing Single-Event Aircraft Noise Data for Specific Engine Power and Meteorological Conditions for Use with USAF (United States Air Force) Community Noise Model (NOISEMAP).

Abstract: : The Air Force community noise prediction model (NOISEMAP) is used to describe the aircraft noise exposure around airbases and thereby aid airbase planners to minimize exposure and prevent community encroachment which could limit mission effectiveness of the installation. This report documents two computer programs (OMEGA 10 and OMEGA 11) which were developed to prepare aircraft flight and ground runup noise data for input to NOISEMAP. OMEGA 10 is for flight operations and OMEGA 11 is for aircraft ground runups. All routines in each program are documented at a level useful to a programmer working with the code or a reader interested in a general overview of what happens within a specific subroutine. Both programs input normalized, reference aircraft noise data; i.e., data at a standard reference distance from the aircraft, for several fixed engine power settings, a reference airspeed and standard day meteorological conditions. Both programs operate on these normalized, reference data in accordance with user-defined, non-reference conditions to derive single-event noise data for 22 distances (200 to 25,000 feet) in a variety of physical and psycho-acoustic metrics. These outputs are in formats ready for input to NOISEMAP. (Author)

Airport related residential acoustical insulation demonstration project

Abstract: Faced with making a choice between acquiring residences and clearing the property versus acoustically insulating them, the St. Louis Airport Authority undertook a demonstration project to develop firm data on the cost and value of retrofit acoustical insulations for residences in the airport environs. Six wood frame homes, typical of most of the housing stock in the area, were modified using standard available construction techniques and materials in an effort to improve their insulation against aircraft noise. Noise exposure measurements were made before and after modifications. This paper reports the details of the retrofit modifications employed and their effectiveness in reducing exposure to aircraft noise.

Acoustic measurements of the X-wing rotor

Abstract: Noise measurements of a stoppable X-wing rotor system model, tested in the Ames 40- by 80-foot wind tunnel, are summarized. Performance, control system stability, and noise of the model were investigated at various forward speeds, tip speeds, collective blade angles, jet blowing velocities, and model attack angles. The model was tested in the rotating wing helicopter configuration, in the fixed wing configuration, and in wing configurations between the two. Noise data obtained in the helicopter configuration at the two highest tip speeds (Mach 0.44 and 0.47) and at wind tunnel speeds below 140 knots are reported. Test configuration and performance information are included. General acoustic measurements (dB, dBA, and PNdB) at six microphone locations are presented for all conditions under which the background noise was below the model noise. More specific measurements (1/3-octave and blade passage frequency harmonic levels) are presented for selected conditions. Graphs of dBA and 1/3-octave spectra, which show the noise trends as functions of operating condition, are included. The noise depends mainly on the jet blowing velocity. The noise levels were highest at moderate jet blowing velocities, less at the highest velocity, and lowest with no blowing at all.

The quiet short-haul research aircraft

Abstract: The design concepts, performance capabilities, and projected applications of the Quiet Short-Haul Research Aircraft (QSRA) are discussed. The propulsive lift system of the QSRA provides the lift required for short field operations at low community noise levels. This system consists of four high bipass ratio, geared turbofan engines mounted so that the engine exhaust flows across the upper surface of the wing (upper surface blowing). Large specially shaped flaps behind each engine control the direction of the flow for each phase of flight. A 95 passenger short haul transport based on this technology could operate out of a 2500 foot runway with a combined takeoff and landing 90 EPNdB footprint area of 2.7 sq mi.